1. Field of the Invention
This invention relates to electrosurgery apparatus. In particular, the present invention relates to a safety device which monitors the area of contact between the patient and the return electrode.
2. Description of the Prior Art
Although electrosurgery apparatus has been known for many years, it has grown in importance and has achieved wide use in recent years. A typical electrosurgery apparatus includes a current generator which supplies a high frequency electric current to an active electrode. This active electrode is normally a pencil shaped instrument which is held by the surgeon. A return electrode (which is sometimes called a "dispersive electrode" or "patient plate") is connected to the current generator with a return electrical conductor and makes electrical contact with the patient. In operation, therefore, the high frequency electric current flows from the current generator through the active electrode, through the patient's body to the return electrode, where it is returned to the current generator through the return electrical conductor.
Because of the small contact area at the active electrode--body interface, a high current density exists which causes a surgical effect. In contrast, the return electrode is large in size in comparison to the active electrode, and therefore the current density at this interface is low enough that no surgical effect occurs.
If a portion of the return electrode is not in contact with the patient, the effective area of the return electrode is reduced, thereby resulting in increased current density at the return electrode--patient interface. This increased current density may result in injury to the patient.
In the prior art, this hazard potential has been dealt with in a number of ways. The most common approach is to monitor the current going to the active electrode and the current from the return electrode and compare the magnitude of the two. If the active current is greater than the return current by a given amount, an alarm signal is produced. This approach assumes that if the active conductor current is greater than the return conductor current by a given amount, the return current is taking some alternate path which may be hazardous to the patient. Examples of other systems which sense return current are shown in the following U.S. Pat. Nos.: Estes 3,601,126; Ikuno et al 3,897,787; Bross 3,913,583; Newton et al 4,094,320; Ikuno et al 4,102,341; and Meinke et al 4,114,623.
The basic problem with this approach is that the criteria for alarm of the monitoring system does not relate directly to the area of contact of the return electrode to the patient. For example, a return electrode with only a small area of contact to the patient may conduct all of the return current, and therefore no alarm signal would be produced even though the current density may be high enough to burn the patient. Another problem with this approach is that the current must go through alternate paths to establish an alarm condition, therefore the patient is subjected to a hazardous condition at the time of alarm. Still another problem with this approach is that because of leakage currents (e.g. capacitive coupling of active and return electrode leads and of patient to ground), the disparity between the active and return currents required to signal an alarm must be larger than a safe amount.
Another prior art approach, as described by Blackett in U.S. Pat. No. 4,122,854, is to monitor the potential of the return electrical conductor at the generator with respect to a ground reference. If the return path is open, current will return through alternate paths causing the potential of the return electrical conductor to increase with respect to ground reference. This apparatus has the same deficiencies as described above. First, the monitoring system is not sensitive to the effective return electrode contact area. Second, at the time of an alarm condition the patient is subjected to a hazardous condition. Another patent showing a similar approach is Gonser U.S. Pat. No. 3,905,373.
Still another prior art approach, as described by Bolduc in U.S. Pat. No. 3,642,008, is to section the return electrode and monitor the continuity between sections. The theory behind this approach is that if the return electrode is in contact with the patient, the patient's body will provide the connection between sections. An alarm condition exists if no contact is made between sections. This approach, however, does not relate to the area of contact between return electrode and patient, in that only a portion of each section need be in contact with the patient to satisfy the monitor. Also, the conductive gel that is normally used to interface the return electrode to the patient can provide conductive connections between sections without the return electrode being in contact with the patient.
Still another prior art approach limits the amount of current to which the return electrode-to-patient interface can be subjected. As with all prior art monitor methods, however, the integrity of return electrode to patient contact is not monitored.
Other U.S. Pat. Nos. showing electrosurgery apparatus include Oringer 3,812,858; Andrews et al 3,923,063; Gonser 3,929,137; Schneiderman 4,051,855; Tankman et al 4,109,223; Gonser 4,123,673; and Holsinger 3,699,389.
Examples of return electrodes are shown in the following U.S. Pat. Nos.: Bolduc 3,699,968; Patrick, Jr. et al 3,848,600; Justus et al, 3,895,635; Bolduc 3,960,141; Twentier 4,088,133; Kaufman 4,092,985; Williams 4,117,846; and Esty et al 4,166,465.
The following U.S. Pat. Nos. relate to safety devices for medical electronic devices or equipment, but are not concerned with electrosurgery: Ogle 3,605,728; Bass 3,889,184; Niemi 4,068,699; Yulk 4,102,347; and Hihara et al 4,102,348.